Telephone transmitter circuit



Jan. 4, 1944. R. N. HARMON TELEPHONE TRANSMITTER CIRCUIT Filed Oct. 22, 1942 QLNXLUU E! ESOurce lll INVENTOR Ralph M Hbrmon.

WITNESSES: Wd@/ 3 ATTOR Y Patented Jan. 4, 1944 TELEPHONE TRANSMITTER CIRCUIT Ralph N. Harmon, Baltimore, Md.. 'assignor to Westinghouse Electric\& Manufacturing Com- ..pany, East Pittsburzhla., a corporation oi.

Pennsylvania Application October 22, 1942, Serial No. 462,985

3 Claims.

My invention relates to carrier current telephone transmitter circuits and, more particularly, to arrangements for improving the electrical efficiencies of such devices.

In carrier current telephone transmitters the amplitude of the radio frequency wave which is sent out into space varies from instant to instant in synchronism with the audio frequencies of the sound being transmitted; that is to say, if the instantaneous values of the radio frequency current were plotted against time, the maximum values of successive radio frequency waves will fluctuate in value so that if enveloped curves be drawn for these radio frequency waves, it will fluctuate in correspondence with the audio frequency waves of sound. Fig. 1 illustrates such plotted curves, the carrier waves appearing in full line and the envelope in chain-dot. When the sound waves are at an instantaneous maximum the enveloped curve will be at a maximum displacement A from the time and when, a halfcycle sound frequency later, the sound wave reaches its negative maximum, the enveloped curve will approach more closely to the time axis as at B. As the sound waves increase in intensity the minimum on the envelope wave approaches more closely to the time axis; but since the amplitude of the radio frequency waves can obviously not fall below zero, the minimum of the enveloped wave cannot cross the time axis. This is merely another way of saying that only when the loudest sounds are being transmitted is the radio frequency wave being modulated 100%; and for less intense sounds the radio frequency wave is always a sine wave of finite amplitude, as it is for instance at B in Fig. 1. Thus, except in those rare moments when the sound being transmitted is at its maximum intensity, the

radio frequency waves which are being sent out are of finite value (for example, the value of B in Fig. 1) and so represent the expenditure of .a finite quantity of electric power.

modulation is nearly always less than one hundred per cent; and this means waste of electric power.

It occurred to me that an arrangement could be made in which the radio frequency waves were emitted in successive groups having a constant amplitude but separated from each other by intervals in which the waves had zero amplitude, the duration of the successive groups being proportional to the instantaneous amplitude of the sound waves being carried. In such an arrangement the power transmitted would always be directly proportional to the instan- Inshort,

taneous intensity of the sound wave and there would be no such waste of power as has been shown above to be characteristic of the conventional methods of modulating the radio frequency waves.

It is accordingly an object of my invention to provide a carrier current telephone transmitter which shall have a higher electrical efficiency than did such telephone transmitters of the prior art.

It is another object of my invention to provide a carrier current telephone transmitter in which the amplitude of the carrier. frequency waves is constant while their average amplitude varies from instant to instant in correspondence with the intensity of the sound modulations.

It is a still further object of my invention to provide a carrier current telephone system in which the modulation corresponding to sound operates through the duration of successive groups of radio frequency waves.

Other objects of my invention will become ap parent in reading the following description taken in connection with the drawing in which:

Figure 1 is a graph already used and showing the fluctuations of intensity of a modulated radio frequency current; and

Fig. 2 is a schematic diagram of one embodiment of my invention.

meral I designates a simple type of radi frequency amplifier comprising a controlgelectrqde and having an output circuit 2 coupled to the primary 3 of a radio frequency transformer, the primary 3 being bridged by a variable capacitor 4 and connectedthrough a capacitor 5 to the anode 6 of the amplifier tube I. The control electrode 1 of the tube I is supplied with carrier 4 frequency input power of constant amplitude from a carrier frequency source 8 which may be of any suitable type; for instance, a quartz-crystal controlled oscillation generator. The amplifier I is arranged to be plate modulated through an inductance coil 9 which is supplied with audio frequency energy for modulation purposes from an audio frequency input circuit ll through the circuit arrangements about to be described. The plate electrode 6 is supplied with direct current through the inductance coil 9 and Heising choke-coil l2 from a direct current source l3 by way of a grid controlled electron tube M which may be alluded to as performing the functions of an electronic switch or circuit interrupter. r

The tube 14 acts to supply plate current to the amplifier tube I in successive pulses of a variable duration: separated by intervals, and constitutes the arrangement for subdividing the output waves of carrier frequency power into successive groups separated from each other by time intervals, all as described in the preceding general description of my invention. q

The length of the successive periods durin which the electronic switch is open to apply plate voltage to the amplifier I, and of the alternate periods during which the electronic switch interrupts the circuit from the direct current source I3 thereby reducing the plate voltage of the amplifier I to zero, is determined by a pulse generator comprising two grid controlled electronic tubes I-5 and it which may be connected up to constitute'a multivibrator. Plate voltage is supplied to the two tubes I5 and I6 from a. direct current source I! through a pair of inductors I8, 19, the cathodes of the two tubes I5, I6 being connected to the negative terminal of the source I1. The anode of the tube I5 is connected through a capacitor 2| to the control electrode 22 of the tube I6 and the anode of thetube I6 is connected through a capacitor 23 to the control electrode 24 of the tube I 5.

A periodic voltage is impressed relative to its cathode on the grid 22 of the tube I6 by an oscillator 25, and the grid 22 is likewise connected through the resistor 26 and a negative bias battery 21 to the cathode of the tube I6. The oscillator 25 causes the multivibrator comprising the tubes I5 and It to supply periodic pulses of voltage of a fixed frequency and amplitude to the input of an amplifier 28 which in turn impresses its output through a transformer 29 between the control electrode 3| and the cathode 32 of the tube I l. The control electrode 3|,is biased relative to the cathode 32 by a source of direct current voltage 33 in a well known manner.

The control electrode 24 of the tube I5 is connected to its cathode through a capacitor 34'and is likewise connected through an inductor 35 to the output of a full wave rectifier on which the speech or other audio signal with which it is desired to modulate the amplifier is impressed. The full wave rectifier comprises a secondary winding 36 having its midpoint connected to the cathode of the tube l5 and having its respective end terminals connected through a pair of rectifiers 31, 38 to the inductor 35. A capacitor 39 is connected between the common terminals of the rectifier 37, 38 and the cathode of the tube l5. It will usually be found convenient in actual practice to ground the cathodes f the tubes I5, I6 and when this is done the connections above mentioned of other portions of the circuit to said cathodes may be made through ground as illus-- trated in Fig. 2. 4

Cooperating with the secondary winding 36 is a primary winding 4| .on which is impressed the audio input II, and this audio input may likewise be connected as shown in the drawing between the cathode of the tube land the inductor 9.

Audio signals such as speech incident on the primary winding M are rectified through the circuit connected to the secondary winding 36 and are impressed through the inductor 35 and capacitor 34 on the control electrode 24 of the tube I5. The effect of this is to vary the length of the pulses of voltage impressed by the multi vibrator comprising the tubes I5, IE on the control electrode 3i of the tube I4, thereby varying the length ofthe successive periods in which the latter permits plate current to be impressed by the source I3 on the anode 8 of the amplifier I. Since the time separating successive pulses of the multivibrator I5. I5 is a constant quantity set by the I frequency of the oscillator 28 an increase of the 5 length of the pulse or voltage which the electronicswitch I4 impresses on the anode of ampliner I results in a decrease of the ensuing time during which the electronic switch I,4 cuts ofl' voltage from the anode of amplifier I. The irequency of the oscillator 25 is made so much larger than the highest frequencies of any sound which it is desired to transmit that many Pulses of anode voltage are impressed through electronic switch I I on the anode of tube I during even onehalf cycle of the highest frequency sound wave. The frequency of the source 8 is, in turn, made large compared with the frequency of the oscillator 25. In consequence of these frequency relations, the output of the amplifier 1 consists of successive groups of waves of the frequency of the source 8 separated by time intervals or gaps; but by reason of the action of the rectified audio signal impressed on the grid 24 of the tube I5 the number of waves of the frequency of source 8 comprised in each such group is proportional from. instant to instant to the intensity of the sound being transmitted, and these subdivisions of time are sufficiently short, because of the high frequency of the oscillator 25, so that the sinusoidal variations of even the highest pitch in the sound waves is closely imitated by successive variations in length of the wave groups which the amplifier I produces as above described. These successive groups of waves having a frequency of the source 8 are, as has already been said, delivered to the output circuit on the secondary winding 2.

While the amplitude of the waves of the frequency of source 8 composing each little group 40 is substantially constant, and only the number of waves in the group varies, the average power output from the amplifier I will be proportional from moment to moment to the number of waves in the group; and since the latter is, as previously stated, proportional from instant to instant to the intensity of the sound being reproduced, the average value over longer periods of time, of the power output of the tube I, will be strictly proportional to the average value of the energy in the sound wave actuating the audio input I'I during the same length of time. There is, accordingly, no waste of radio frequency power incident 1 to modulating the output of the amplifier I, as is the case in prior art systems of modulation and the efliciency of my modulation system is accordingly a substantial improvement over that of the prior art.

While I have described a specific embodiment of my invention, the principles thereof will be applicable in other ways which will be evidentto those skilled in the art.

I claim as my invention:

1. In combination with an electrical discharge tube having a source of carrier frequency voltage connected to its control electrode, an output circuit connected between its principal electrodes in combination with a source of direct current in series with a circuit interrupter comprising an electrical discharge tube having a control electrode, means for impressing voltage pulses of constant periodicity but variable width on the last mentioned control electrode, and means for varying the width of said voltage pulses in substantial proportionality to the instantaneous intensity of a modulating signal.

2. In combination with an electrical'discharge tube having a source of carrier frequency voltage connected to its control electrode, an output circuit connected between its principal electrodes in combination with a source of direct current in series with a circuit interrupter, means for periodically closing and opening said circuit interrupter, and means for causing the duration of successive closures of said circuit interrupter to be proportional to the amplitude in their periodic cycle of sound waves which it is desired to transmit.

3. In combination with an electrical discharge tube having a source of carrier frequency voltage connected to its control electrode, an output circuit connected between its principal electrodes in 15 combination with a source of direct current in series with a circuit interrupter comprising an electrical discharge tube having a control electrode, 

